1. Field of the Invention
The present invention relates to an apparatus and method for acquiring focusing information, in which focusing is detected from several images different in unclearness, with reference to the command values and the database of spread parameters, the command values having been supplied to a drive unit to establish a focusing state already stored
2. Description of the Related Art
U.S. Pat. No. 4,965,840 discloses a focusing detecting method using a DFD (depth-from-defocus). This method acquires luminance information at two sites having different optical-path lengths. Data representing a plurality of images that differ in unclearness is processed, thereby calculating a spread parameter and determining focusing.
“Spread parameter” is a representative value representing the unclearness of luminance information. This value is correlated to the distribution of point-spread functions in an optical system. “Point spread function” is a function that represents the spread of light that takes place when an ideal point image passes through the optical system.
“Luminance information” is a numerical value for the amount of light per unit area in an optical image. The luminance information is acquired by a photoelectric transducer (1 pixel) that has a photoelectric conversion function, or by an image sensor than comprises a plurality of photoelectric transducers. The luminance information acquired by a monochrome image sensor is a sensor signal information per se. The luminance information acquired by a color image sensor is signal information items that the color filters obtain for the respective bands. It may be signal information items for R, G and B, respectively. Otherwise, it may be information obtained by synthesizing the signal information items. The signal information is not limited to a particular format. It may be ordinary image information acquired by an area type sensor and representing a two-dimensional image, a combination of image information items linearly arranged, one-dimensional image information acquired by a line-type sensor, or one-point information acquired by a one-segment imaging element.
U.S. Pat. No. 4,965,840 discloses a focusing detecting method using DFD. The steps of this method will be explained briefly. The data processing performed in these steps will not be described here, because it is described in detail in U.S. Pat. No. 4,965,840.
In the focusing detecting method using DFD, at least two luminance information items for determining the focusing are acquired from the same object, the same position and the same line of sight, by changing at least one photographing parameter that influences the unclearness of the image photographed. Several photographing parameter are available. They are the position of the focus lens, opening of the diaphragms, focal length, and the like. Here, it will be descried only how the position of the focus lens is changed. It limits, when changing only the position of a focus lens, and this explanation explains.
In this focusing detecting method using DFD, the focus lens is moved to a first predetermined position and then to a second position, in order to change the length of the optical path between an imaging unit and an object. First luminance information and second luminance information are thereby acquired. Normalization is performed on these luminance information items, thus normalizing the image magnification, luminance distribution, and the like. Any part of the luminance information for determining whether the part is well focused or not is selected, as needed. This selection is performed for only one luminance information item acquired. In the part of the other luminance information item, a corresponding part is selected according to the selected part of the first-mentioned luminance information. The first and second items of luminance information items contain electrical noise. Therefore, preliminary operations are performed on those parts of the first and second items of luminance information, the preliminary operations including an operation for noise elimination and an operation for calculating a spread parameter. The results of the preliminary operations are combined. Thereby, the spread parameter corresponding to the first or second items of luminance information are calculated.
The spread parameter thus calculated is related to the command value for driving an actuator that moves the focus lens to achieve focusing in accordance with this parameter. A table showing the correspondence of the parameter and the command value has been already prepared. Hence, if the spread parameter acquired is checked against the correspondence table, the command value for driving the actuator can therefore be generated.